線程同步的方法(線程同步的方法有哪些)

線程同步的方法有哪些?在linux下,系統提供了很多種方式來實現線程同步,其中最常用的便是互斥鎖、條件變量和信號量這三種方式,可能還有很多伙伴對于這三種方法都不熟悉,下面就給大家詳細介紹下。

Linux下實現線程同步的三種方法:

一、互斥鎖(mutex)

通過鎖機制實現線程間的同步。

1、初始化鎖。在Linux下,線程的互斥量數據類型是pthread_mutex_t。在使用前,要對它進行初始化。

靜態分配:pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

動態分配:int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutex_attr_t *mutexattr);

2、加鎖。對共享資源的訪問,要對互斥量進行加鎖,如果互斥量已經上了鎖,調用線程會阻塞,直到互斥量被解鎖。

int pthread_mutex_lock(pthread_mutex *mutex);

int pthread_mutex_trylock(pthread_mutex_t *mutex);

3、解鎖。在完成了對共享資源的訪問后,要對互斥量進行解鎖。

int pthread_mutex_unlock(pthread_mutex_t *mutex);

4、銷毀鎖。鎖在是使用完成后,需要進行銷毀以釋放資源。

int pthread_mutex_destroy(pthread_mutex *mutex);

#include <cstdio>#include <cstdlib>#include <unistd.h>#include <pthread.h>#include "iostream"using namespace std;pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;int tmp;void* thread(void *arg){cout << "thread id is " << pthread_self() << endl;pthread_mutex_lock(&mutex);tmp = 12;cout << "Now a is " << tmp << endl;pthread_mutex_unlock(&mutex);return NULL;}int main(){pthread_t id;cout << "main thread id is " << pthread_self() << endl;tmp = 3;cout << "In main func tmp = " << tmp << endl;if (!pthread_create(&id, NULL, thread, NULL)){cout << "Create thread success!" << endl;}else{cout << "Create thread failed!" << endl;}pthread_join(id, NULL);pthread_mutex_destroy(&mutex);return 0;}//編譯:g++ -o thread testthread.cpp -lpthread

二、條件變量(cond)

與互斥鎖不同,條件變量是用來等待而不是用來上鎖的。條件變量用來自動阻塞一個線程,直到某特殊情況發生為止。通常條件變量和互斥鎖同時使用。條件變量分為兩部分: 條件和變量。條件本身是由互斥量保護的。線程在改變條件狀態前先要鎖住互斥量。條件變量使我們可以睡眠等待某種條件出現。條件變量是利用線程間共享的全局變量進行同步的一種機制,主要包括兩個動作:一個線程等待“條件變量的條件成立”而掛起;另一個線程使“條件成立”(給出條件成立信號)。條件的檢測是在互斥鎖的保護下進行的。如果一個條件為假,一個線程自動阻塞,并釋放等待狀態改變的互斥鎖。如果另一個線程改變了條件,它發信號給關聯的條件變量,喚醒一個或多個等待它的線程,重新獲得互斥鎖,重新評價條件。如果兩進程共享可讀寫的內存,條件變量可以被用來實現這兩進程間的線程同步。

1、初始化條件變量。

靜態態初始化,pthread_cond_t cond = PTHREAD_COND_INITIALIER;

動態初始化,int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *cond_attr);

2、等待條件成立。釋放鎖,同時阻塞等待條件變量為真才行。timewait()設置等待時間,仍未signal,返回ETIMEOUT(加鎖保證只有一個線程wait)

int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);

int pthread_cond_timewait(pthread_cond_t *cond,pthread_mutex *mutex,const timespec *abstime);

3、激活條件變量。pthread_cond_signal,pthread_cond_broadcast(激活所有等待線程)

int pthread_cond_signal(pthread_cond_t *cond);

int pthread_cond_broadcast(pthread_cond_t *cond); //解除所有線程的阻塞

4、清除條件變量。無線程等待,否則返回EBUSY

int pthread_cond_destroy(pthread_cond_t *cond);

[cpp] view plain copy#include <stdio.h>#include <pthread.h>#include "stdlib.h"#include "unistd.h"pthread_mutex_t mutex;pthread_cond_t cond;void hander(void *arg){free(arg);(void)pthread_mutex_unlock(&mutex);}void *thread1(void *arg){pthread_cleanup_push(hander, &mutex);while(1){printf("thread1 is running\n");pthread_mutex_lock(&mutex);pthread_cond_wait(&cond, &mutex);printf("thread1 applied the condition\n");pthread_mutex_unlock(&mutex);sleep(4);}pthread_cleanup_pop(0);}void *thread2(void *arg){while(1){printf("thread2 is running\n");pthread_mutex_lock(&mutex);pthread_cond_wait(&cond, &mutex);printf("thread2 applied the condition\n");pthread_mutex_unlock(&mutex);sleep(1);}}int main(){pthread_t thid1,thid2;printf("condition variable study!\n");pthread_mutex_init(&mutex, NULL);pthread_cond_init(&cond, NULL);pthread_create(&thid1, NULL, thread1, NULL);pthread_create(&thid2, NULL, thread2, NULL);sleep(1);do{pthread_cond_signal(&cond);}while(1);sleep(20);pthread_exit(0);return 0;}

#include <pthread.h>#include <unistd.h>#include "stdio.h"#include "stdlib.h"static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;struct node{int n_number;struct node *n_next;}*head = NULL;static void cleanup_handler(void *arg){printf("Cleanup handler of second thread./n");free(arg);(void)pthread_mutex_unlock(&mtx);}static void *thread_func(void *arg){struct node *p = NULL;pthread_cleanup_push(cleanup_handler, p);while (1){//這個mutex主要是用來保證pthread_cond_wait的并發性pthread_mutex_lock(&mtx);while (head == NULL){//這個while要特別說明一下,單個pthread_cond_wait功能很完善,為何//這里要有一個while (head == NULL)呢?因為pthread_cond_wait里的線//程可能會被意外喚醒,如果這個時候head != NULL,則不是我們想要的情況。//這個時候,應該讓線程繼續進入pthread_cond_wait// pthread_cond_wait會先解除之前的pthread_mutex_lock鎖定的mtx,//然后阻塞在等待對列里休眠,直到再次被喚醒(大多數情況下是等待的條件成立//而被喚醒,喚醒后,該進程會先鎖定先pthread_mutex_lock(&mtx);,再讀取資源//用這個流程是比較清楚的pthread_cond_wait(&cond, &mtx);p = head;head = head->n_next;printf("Got %d from front of queue/n", p->n_number);free(p);}pthread_mutex_unlock(&mtx); //臨界區數據操作完畢,釋放互斥鎖}pthread_cleanup_pop(0);return 0;}int main(void){pthread_t tid;int i;struct node *p;//子線程會一直等待資源,類似生產者和消費者,但是這里的消費者可以是多個消費者,而//不僅僅支持普通的單個消費者,這個模型雖然簡單,但是很強大pthread_create(&tid, NULL, thread_func, NULL);sleep(1);for (i = 0; i < 10; i++){p = (struct node*)malloc(sizeof(struct node));p->n_number = i;pthread_mutex_lock(&mtx); //需要操作head這個臨界資源,先加鎖,p->n_next = head;head = p;pthread_cond_signal(&cond);pthread_mutex_unlock(&mtx); //解鎖sleep(1);}printf("thread 1 wanna end the line.So cancel thread 2./n");//關于pthread_cancel,有一點額外的說明,它是從外部終止子線程,子線程會在最近的取消點,退出//線程,而在我們的代碼里,最近的取消點肯定就是pthread_cond_wait()了。pthread_cancel(tid);pthread_join(tid, NULL);printf("All done -- exiting/n");return 0;}

三、信號量(sem)

如同進程一樣,線程也可以通過信號量來實現通信,雖然是輕量級的。信號量函數的名字都以“sem_”打頭。線程使用的基本信號量函數有四個。

1、信號量初始化。

int sem_init (sem_t *sem , int pshared, unsigned int value);

這是對由sem指定的信號量進行初始化,設置好它的共享選項(linux 只支持為0,即表示它是當前進程的局部信號量),然后給它一個初始值VALUE。

2、等待信號量。給信號量減1,然后等待直到信號量的值大于0。

int sem_wait(sem_t *sem);

3、釋放信號量。信號量值加1。并通知其他等待線程。

int sem_post(sem_t *sem);

4、銷毀信號量。我們用完信號量后都它進行清理。歸還占有的一切資源。

int sem_destroy(sem_t *sem);

#include <stdlib.h>#include <stdio.h>#include <unistd.h>#include <pthread.h>#include <semaphore.h>#include <errno.h>#define return_if_fail(p) if((p) == 0){printf ("[%s]:func error!/n", __func__);return;}typedef struct _PrivInfo{sem_t s1;sem_t s2;time_t end_time;}PrivInfo;static void info_init (PrivInfo* thiz);static void info_destroy (PrivInfo* thiz);static void* pthread_func_1 (PrivInfo* thiz);static void* pthread_func_2 (PrivInfo* thiz);int main (int argc, char** argv){pthread_t pt_1 = 0;pthread_t pt_2 = 0;int ret = 0;PrivInfo* thiz = NULL;thiz = (PrivInfo* )malloc (sizeof (PrivInfo));if (thiz == NULL){printf ("[%s]: Failed to malloc priv./n");return -1;}info_init (thiz);ret = pthread_create (&pt_1, NULL, (void*)pthread_func_1, thiz);if (ret != 0){perror ("pthread_1_create:");}ret = pthread_create (&pt_2, NULL, (void*)pthread_func_2, thiz);if (ret != 0){perror ("pthread_2_create:");}pthread_join (pt_1, NULL);pthread_join (pt_2, NULL);info_destroy (thiz);return 0;}static void info_init (PrivInfo* thiz){return_if_fail (thiz != NULL);thiz->end_time = time(NULL) + 10;sem_init (&thiz->s1, 0, 1);sem_init (&thiz->s2, 0, 0);return;}static void info_destroy (PrivInfo* thiz){return_if_fail (thiz != NULL);sem_destroy (&thiz->s1);sem_destroy (&thiz->s2);free (thiz);thiz = NULL;return;}static void* pthread_func_1 (PrivInfo* thiz){return_if_fail(thiz != NULL);while (time(NULL) < thiz->end_time){sem_wait (&thiz->s2);printf ("pthread1: pthread1 get the lock./n");sem_post (&thiz->s1);printf ("pthread1: pthread1 unlock/n");sleep (1);}return;}static void* pthread_func_2 (PrivInfo* thiz){return_if_fail (thiz != NULL);while (time (NULL) < thiz->end_time){sem_wait (&thiz->s1);printf ("pthread2: pthread2 get the unlock./n");sem_post (&thiz->s2);printf ("pthread2: pthread2 unlock./n");sleep (1);}return;}

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